This invention relates generally to a heap bioleaching process for the recovery of one or more metals from an ore.
The invention is described hereinafter with particular reference to the recovery of copper from a low grade marginal ore (eg. less than 0.7% copper) containing refractory primary sulphide minerals such as chalcopyrite. This however is only by way of example and the principles of the invention can be used in other appropriate circumstances for the recovery of different metals from different ores.
The heap bioleaching of copper is a microbial (bacterial and archaeal) mediated leaching process wherein:                the microorganisms oxidise ferrous iron to ferric iron;        the ferric iron facilitates an initial attack on the sulphide while sulphur oxidising microorganisms further oxidise the reduced sulphur species to sulphate;        the microbial oxidation of such sulphur species results in the release of heat;        the heat generated has important implications for the subsequent leaching process, particularly for primary copper minerals such as chalcopyrite which do not leach well at low temperatures (below 45° C.) and which require higher temperatures of up to 65° C. in order to achieve a satisfactory leaching rate; and        acid is generated which is important for the leaching process and for maintaining the copper in solution.        
In order to achieve elevated heap temperatures conducive to chalcopyrite heap leaching sequential populations of bioleaching microorganisms are required. This is necessary because microorganisms which predominate at ambient temperature, at heap start up, are not able to grow and contribute to the bioleaching process at elevated temperatures. For example microbial strains with a temperature optimum of 35° C. would have a relatively low activity at 45° C. and above while strains with a temperature optimum at 65° C. would have a relatively low activity at 45° C.
As is known in the art oxygen and carbon dioxide are supplied in the form of air to a heap. The oxygen is required for microbiological and chemical oxidation reactions while the carbon dioxide is required as a carbon source for the microorganisms.
The use of sequential microbial populations with increasing temperature optima is required to raise the temperature of the heap from ambient to a value at which chalcopyrite leaching can take place. The temperature increase results from heat which is generated by the bacteria and archaea oxidising sulphur.
It is known to assess bioleaching activity within a heap by monitoring the conversion rate of ferrous iron to ferric iron, in addition to the copper recovery. An indication of the rate of ferrous oxidation can relatively easily be obtained from the pregnant liquor solution which drains from the heap, either by measuring the ratio of ferrous iron to ferric iron in solution or by monitoring the redox potential, which is a function of the ferrous to ferric ratio.